The main function of lipoproteins in plasma is to transport lipids, such as cholesterol and triglycerides. For transport in plasma, cholesterol, normally as cholesteryl esters, and the triglycerides are included into lipoprotein particles in which they form a hydrophobic core. The core is surrounded by a surface coat containing phospholipids, unesterified cholesterol and proteins called apolipoproteins. The latter are responsible for the lipid transport, and in addition, some may interact with many of the enzymes involved in lipid metabolism. To date, at least nine apolipoproteins have been identified: A-I, A-II, A-IV, B, C-I, C-II, C-III, D and E.
There are four major classes of lipoproteins: chylomicrons (CM), very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins. Of these, HDL is directly involved in the removal of cholesterol from peripheral tissues, carrying it back either to the liver or to other lipoproteins, by a mechanism known as "reverse cholesterol transport" (RCT).
The "protective" role of HDL has been confirmed in a number of studies. Recent studies directed to the protective mechanism(s) of HDL have been focused on apolipoprotein A-I (ApoA-I), the major component of HDL. High plasma levels of ApoA-I are associated with a reduced risk of CHD and presence of coronary lesions.
The apolipoprotein A-IMilano (ApoA-IM) is the first described molecular variant of human ApoA-I (Franceschini et al. (1980) J. Clin. Invest. 66: 892-900). It is characterized by the substitution of Arg 173 with Cys 173 (Weisgraber et al. (1983) J. Biol. Chem. 258: 2508-2513). The mutant apolipoprotein is transmitted as an autosomal dominant trait and 8 generations of carriers have been identified (Gualandri et al. (1984) Am. J. Hum. Genet. 37: 1083-1097). The status of a ApoA-IM carrier individual is characterized by a remarkable reduction in HDL-cholesterol level. In spite of this, the affected subjects do not apparently show any increased risk of arterial disease. Indeed, by examination of the genealogical tree it appears that these subjects may be "protected" from atherosclerosis.
Apolipoprotein E (ApoE) is a ligand for the LDL receptor. As a result, ApoE plays an important role in cholesterol metabolism. In addition, ApoE is involved in the hepatic clearance of chylomicron remnants.
Several methods have been proposed for purifying ApoA and ApoE from plasma. One of the most common ways to purify apolipoprotein A-I is to use ultracentrifugation in order to isolate high density lipoproteins (HDL) followed by a separation of Apo A-I from the HDL-particle. There are several different ways to purify Apo A-I from HDL, including solvent extraction. Ultracentrifugation is a very time-consuming method and it is not suitable for large scale isolation. Methods using plasma as starting material and which do not include ultracentrifugation have been described, for example chromatographic purification (Ross S. E. et al, Rapid chromatographic purification of apolipoproteins A-I and A-II from human plasma, Analytical Biochemistry 149, p. 166-168 (1985)), and purification using gel-filtration HPLC (Tricerri A. et al, A rapid and efficient procedure for the purification of human apolipoprotein A-I using gel-filtration HPLC, IJBC, 1, p. 159-166 (1994)). Other methods which use fractions from cold ethanol fractionation as starting material have also been published (Peitsch et al, A purification method for apolipoprotein A-I and A-II, Analytical Biochemistry, 178, p. 301-305 (1989)).
EP-A-0329 605 to Rotkreuzstiftung Zentrallaboratorium Blutspendedienst SRK and Lerch et al, Isolation and properties of apolipoprotein A for therapeutic use, Protides Biol. Fluids, 36, p. 409-416 (1989), relate to preparation of Apolipoproteins from fractions of human blood plasma containing lipoproteins. EP-A-0329 605 and Lerch et al disclose that precipitate B and IV of a cold ethanol fractionation process can be used as starting material for producing ApoA. Use is made of buffers containing unusually high ethanol concentrations (68-96% ethanol), optionally with an organic solvent, for precipitating contaminants. The precipitates are solubilized in guanidine hydrochloride, which is subsequently removed by gel filtration. An anion-exchange chromatography step is included to bind the contaminants, while the ApoA passes through.
JP-A-08003198 to Chemo-Sero-Therapeutics Research Institute relates to preparation of Apolipoprotein A-I from human plasma by incubating apolipoprotein A-I containing plasma with a lower aliphatic alcohol, centrifuging the culture mixture obtained, and applying the resulting supernatant to a hydrophobic chromatography resin in the presence of a lower aliphatic alcohol.
WO-A-93/12143 to Pharmacia & Upjohn (formerly Kabi Pharmacia AB) relates to preparation of the dimer of Apolipoprotein A-IM from blood plasma collected from Apo A-IM carriers as well as from solutions containing Apolipoprotein A-IM produced by recombinant DNA techniques. The dimers can be produced from blood plasma by isolating the high density lipoproteins (HDL) particles and separating the dimer by use of one or more gel filtration steps, or by purifying the monomers on Thiopropyl-Sepharose and thereafter converting said monomers to the dimers.
There are presently several methods known for purifying plasma-derived ApoA and ApoE. There is, however, a need for an additional quick, sensitive and reliable method for preparation of plasma-derived ApoA and ApoE on a pilot-plant and industrial scale. It is the purpose of the present invention to provide such a method.